Why the reference of System.Design.dll not added already in Visual Studio?

When I trying to use "System.Windows.Form.Design" namespace, I have to add reference all the time in new project. Can any tell me about it?

How do I add a project reference code in Visual Studio?

Right click on project --> Add Reference --> Select the reference project from the list.

How do I fix assemble reference error in Visual Studio?

Navigate to the startup project in solution explorer. Right click, properties > Application > Target framework. Change the target framework to anything else. Press Yes for the confirmation dialog.

When you create different projects in Visual Studio it adds by default different BCL assemblies as references. System.Design.dll is not one of them as it is not commonly used by standard apps / developers. That's how the designers decided. You could create your own project type which will already contain the reference and then export it as project template.